Wireless telecommunications systems currently use either terrestrial (ground) based infrastructures or space (satellite) based infrastructures. Terrestrial based systems include radio towers and antennae on tall buildings, mountains, and the like. Also, balloons that are tethered to the ground have been used. Spaced based systems rely on satellites having telecommunications equipment.
Terrestrial based wireless telecommunications systems have been known since the early days of radio, almost a hundred years ago. Their configurations range from simple one-way and two-way radio hookups—to radio and television broadcast networks—to today's sophisticated cellular networks and proposed personal communications networks (PCN).
“Relay stations” are used to send and receive radio transmissions to and from other locations. Because they are on or close to the ground, their radio signals tend on the average to be closer to the horizontal than the vertical. Thus, each relay station can only send and receive signals from a limited distance. The distance that the radio signals can travel is limited because of horizon problems due to the curvature of the earth; line of sight problems due to uneven terrain, trees, and buildings; interference due to other signals or with reflections of the transmitted signal; and attenuation problems due to unwanted absorption of the transmitted signal. To increase the area of coverage, either more powerful equipment must be used, and/or the height of the relay stations must be increased. Increasing power helps to solve the attenuation problem and the interference with other signals problem; but it does not address the horizion, line-of-sight, and interference with relected signal problems. Therefore, it is preferred to increse the height of the relay stations as by putting them on towers, tall buildings and mountain tops. This rolls back the horizon and line-of-sight for the relay station thereby increasing the area that it can cover, and to some extent reduces the attenuation problem and the interference with the reflected signal problem. However, it is not always feasible to place relay stations at optimum locations due to geographic or political factors, or merely because of the inability to obtain permission from a land owner or government.
To some extent these problems are alleviated by wireless telecommunications equipment carried by tethered balloons. However, tethered balloons have their own problems. If the balloons are tethered at low altitudes, their area of coverage will not be any larger than that of a relay station on a tower or tall building making it difficult to justify their cost. Also, since they will be subject to the weather and wind conditions that exist at these altitudes, they are likely to be easily damaged and require frequent replacement.
On the other hand, if they are tethered at altitudes that enable them to relay telecommunications signals over a large enough area to make them economically feasible and to avoid weather conditions, thereby prolonging their life, both the balloons and tethers become hazardous to aircraft and the tethers remain subject to the stress of weather conditions.
Further, it is likely that the tether of a failed balloon will be strewn along hundreds if not thousands or tens of thousands of feet causing damage and risk of injury to property and persons. Additionally, if the tether falls across electric lines there is a risk of fire and power outages.
Accordingly, these disadvantages make tethered ballons unsuitable for use as part of a telecommunications system whose components are to operate for long periods.
To overcome many of the limitations of ground based wireless telecommunications systems, orbital space based telecommunications systems have been constructed using satellite technologies which have evolved since the first days of Sputnik (1957). Satellite systems in geosynchronous orbit (approximately 22,000 miles) have been used for may years with a high degree of reliability. Their prime advantage is their high altitude which enables one satellite to send and receive signals from an area on the earth encompassing hundreds of thousands of square miles. However, satellites are expensive to manufacture, launch and position, either initially or as replacements. Further, because of the cost associated with their manufacture and launch, and the great difficulty in servicing them, extraordinary care must be taken to assure their reliability.
Moreover, because of a satellite's high altitude, there is a delay in radio transmission of about ⅛ of a second in each direction. This significantly limits the satellite's ability to carry and conduct familiar two way (duplex) voice communications. Also, due to its high altitude, its radio transmission equipment requires more power than required by comparable terrestrial systems. This raises costs and affects the size and weight of equipment both on the satellite and on the ground.
When a satellite fails, as assuredly they all must do, either electronically, or by decay of orbit, attempts to recover or repair them are extremely expensive. Further, the attempts, whether or not successful, subject personnel and equipment to the risk of injury or loss. On the other hand, a failed satellite may be left in orbit. It will be another piece of “space junk,” until its orbit decays to the extent that it plunges through the atmosphere toward earth. If it is not fully consumed during the plunge, it may cause damage to persons or property when it strikes the earth.
In an attempt to solve the problems attendant to existing high altitude satellite systems, it has been proposed to orbit the satellites at an altitude of either about 500 miles or at about 5,000 miles. While this will reduce power requirements and transmission delay times, it creates other problems. This is because at these lower altitudes the satellites are not geosynchronous. Therefore, telecommunications signals may be required to be transmitted between several satellites during a particular communication. This is because the circumferential position of each satellite relative to the earth is continuously changing. Therefore, a particular satellite that is over a ground station at the beginning of a communication may orbit to such an extent during the communication that it loses the signal from the ground. To maintain the connection, the signal from the ground will have to be transferred to another satellite that is closer to the ground station. Also, the satellites will have to be programmed to permit this to happen. Thus, very complex routing features will need to be implemented. In addition, members of the industry disagree amongst themselves over optimum altitudes, angles of signal propagation, and how to deal with the doppler shifts. Furthermore, because of their lower altitude, the satellites' orbits will decay at faster rates than the higher altitude satellites so that they and the equipment they carry will need to be replaced more often, again incurring substantial expense.
The problems described could be substantially reduced by a telecommunications infrastructure using long duration, high altitude, recoverable telecommunications stations that can be kept on station and which are located in a sub-orbital plane, and which have the ability to receive telecommunication signals from a ground station and relay them to another similar station or to a further ground station.
Since the propagation of radio signals to and from the relay stations would be nearly vertical; line of sight, reflective interference and attenuation problems would be minimized. This is because there would be less liklihood of tall buildings, trees or terrain to block, relect, or absorb the radio signals. This means that less power would be needed to send a signal a given distance than if it were transmitted horizontally at or near the ground. Further, because the system would operate at altitudes that are less than ten percent of the lowest proposed satellite systems, less power would be required for telecommunications signals with no noticable delay in transmission.
This will create a means for providing relatively low cost, efficient, wireless telecommunications without incurring the economic and physical limitations associated with terrestrial based network infrastructures, tethered balloon systems or orbiting space based network infrastructures.